Ionic impurities are present in all types of liquid crystal displays. These impurities lead to image artefacts such as flicker, cross-talk and image retention and might also affect the lifetime of the display.
In STN-type (Super Twisted Nematic) liquid crystal displays the situation is particularly severe. The highest ion concentrations are in the order of 1020 m−3, resulting in serious problems with cross-talk and image retention.
In LCoS (Liquid Crystal on Silicon) projection displays the initial ion concentration is much lower, typically in the order of 1017 m−3. However, during the lifetime of such a display liquid crystal molecules are photo-chemically dissociated giving rise to an ion concentration increase by multiple orders of magnitude. Ultimately this high ion concentration (typically constituted by F− and Cl−) will lead to a loss of alignment in the liquid crystal layer and end-of-life of the display.
Ion contaminations in the liquid crystal lead to image retention in other AMLCDs (Active Matrix Liquid Crystal Displays) as well. Image sticking problems in AMLCDs have been known for more than 15 years (see e.g. Y. Nanno et al., Characterisation of the sticking effect of TFT-LCDs, Proceedings of the SID, Vol. 31/4 (1990)), and still the problems have never been completely solved. One of the reasons for this is that a virtually undetectable ion concentration (below 1 ion per billion LC molecules) is enough to disturb the electric field in a liquid crystal cell. Despite state-of-art purified materials and clean-room conditioned cell processing, the ion concentrations inside the liquid crystal displays cannot be kept to a low enough level.
Known measures for limiting these problems primarily focus on preventing the in-diffusion of ions from the outside environment into the liquid crystal display area. This is achieved using either a border ring electrode with DC fields (see for example US20020060768, EP1055960, JP2002196355, JP05323336), ion-capturing (adsorbing) materials at the border of the display area (see for example JP2000338505, JP04320211, JP03005723, JP2001201734, JP10177177), or a double wall surrounding the border of the display area (see for example JP06175142).
However, even though these measures indeed reduce the problems related to ion contamination of the liquid crystal the positive effect is still limited.
Border ring electrodes using DC fields will indeed attract incoming ions towards the electrodes. There, some of the ions will adsorb but others will diffuse parallel to the DC electric field. Application of a partly lateral electric DC field (as described in JP2002196355) will make it somewhat more difficult but still not impossible for ions to diffuse into the display area, and it requires relatively high driving voltages. A general drawback using this approach is that it only works for one polarity of ions. In fact, ions having opposite polarity are even forced into the display area. In addition, prolonged application of DC fields can degrade the liquid crystal material.
Ion-capturing materials only capture ions accidentally diffusing towards these materials, there is no mechanism directing the ions towards the ion-capturing materials.
Double walls surrounding the display area will ultimately also be permeable to ions. The idea behind this approach is to prevent ions from diffusing into the liquid crystal when it is filled into the display. Double walls indeed also have the effect of delaying ion diffusion during the lifetime of the display but ultimately ions nevertheless will diffuse into the display area.
Furthermore, as mentioned above undesired ion contamination is not only coming from the surrounding environment but also from the device itself due to diffusion from the alignment layer materials (inside the display area) and/or degradation of the liquid crystal material (by dissociation due to visible or UV light exposure or electric fields). Therefore, even if the ion protection at the borders as proposed in the mentioned prior-art would be totally impenetrable for ions, there would still appear ions in the liquid crystal.
JP2001066580 discloses a different approach, based on wall arrangements inside the display area serving to avoid lateral motion of the ions. It is described that ions move laterally due to potential differences across the LC layer leading to non-uniform ion distributions. The internal wall system described might indeed provide more uniform ion distributions, but does not affect the total ion contamination of the liquid crystal as a whole.